This invention relates to a process for reducing the concentration of dissolved contaminant in a contaminated liquid, and in partitcular to a process for regeneration of contaminated electrolyte for use in an electrolytic cell.
To ensure a good ionic conductivity and a low temperature freezing point, nickel-cadmium electrolytic cells are filled with a potassium hydroxide (KOH) electrolyte solution. Depending upon the storage conditions the KOH electrolyte can contain various amounts of carbonate impurities due to the reaction of atmospheric CO.sub.2 with the KOH. The carbonate content can also increase during the service life of the cells due to the degradation of the separator material, mainly cellophane and nylon. Since it has been recognized that carbonate impurities affect the cranking performance and the low temperature operation of batteries, Canadian Forces specifications restrict carbonate contamination to a maximum of 5% by weight of the electrolyte. If this limit is exceeded, it is advised to exchange the electrolyte. However, the plates and the separator materials in sintered plate, flooded nickel/cadmium cells are highly porous. Much of the electrolyte in the cell is therefore soaked into the pores and clings to the surfaces of these components. Furthermore, the baffle located below the vent cap frequently makes it difficult or impossible to remove all of the electrolyte which is free in the cell. The extent of these limitations may be illustrated by this example. A 22 amperehour cell constructed of dry materials required 130 ml of electrolyte to fill it to the proper level. Later attempts to empty the cell by dumping the electrolyte yielded only 32 ml. Thus, three-quarters of the electrolyte remained in the cell.
In view of the above facts, if the carbonate content in a cell becomes excessive, it can be reduced by completely extracting the electrolyte under high vacuum and replacing it by fresh electrolyte or by repeatedly draining the small amount of electrolyte which can be removed and admixing fresh electrolyte solution with what remains in the cell. In the above cell, for example, if the carbonate content was 12% by weight and 32 ml of solution were removed and replaced by a carbonate free potassium hydroxide solution and neglecting the resulting change in density, the exchange would bring the level down to: 12% (130-32)/130-9% . A second exchange would bring it to 9% (130-32)/130 =6.8% . Third and fourth exchanges would give 5.1% and 3.9% by weight, respectively.
The example assumed that prior to each succeeding exchange, the old and the added electrolyte were uniformly mixed. If this is not so, the number of required exchanges is increased. The second assumption was that the maximum amount of free electrolyte was removed from the cell each time. This is rarely achieved in practice. Unless a good technique is used, it is quite possible that only about one half as much, or less, is actually removed. This can readily double the number of time-consuming exchanges required.
It is thus obvious that the expeditious upgrading of the electrolyte in these batteries demands effective techniques for thorough mixing and for maximum removal of solution each time.
A conventional procedure for mixing the added electrolyte with the retained electrolyte is to completely discharge and recharge the battery. During the discharge much of the electrolyte is drawn into the plate pack and is expelled again during the recharge. However, this procedure normally requires several days.
Considerable improvement in this process was achieved by providing a reservoir of fresh electrolyte in fluid communication with a cell which is put through a discharge - recharge - overcharge cycle. The fresh electrolyte is mixed in the cell with the contaminated electrolyte. The cycle is repeated until analysis of the cell electrolyte shows the required reduced contaminant concentration. This process which is described in French Pat. No. 2,430,671, published Dec. 5, 1980 in the names of Gabriel Coz et al, is quite time consuming and requires attention during the process. For instance, one cycle requires about 8 hours. Another obvious drawback is the associated energy costs in carrying out the process.